Synthesis/Regeneration 34 &nbsp&nbsp(Spring 2004)

Thinking Ecologically

GM Crops Increase Pesticide Use

by Lim Li Ching, Institute for Science in Society

A new report from Dr. Charles Benbrook, director of the Northwest Science and Environmental Policy Center, Idaho, concludes that the 550 million acres of GM corn, soybeans and cotton planted in the US since 1996 have increased pesticide use (herbicides and insecticides) by about 50 million pounds. Benbrook is a respected agricultural economist and was Executive Director of the US National Academy of Sciences Board on Agriculture from 1984 to 1990. The report is the first comprehensive study of the impact of all major commercial GM crops on pesticide use in the US over the first eight years of commercial use, 1996–2003. Most studies to date have only focused on the first three years of GM crop adoption (1996–1998), and no study has estimated impacts in 2002 and 2003.

Benbrook draws on official US Department of Agriculture (USDA) data on pesticide use by crop and state to calculate the overall impact of GM crops on the volume of pesticides applied on corn, soybeans and cotton. These three crops account for nearly all the area planted to GM crops in the US. The analysis focuses on herbicide tolerant (HT) corn, soybeans and cotton; and corn and cotton genetically engineered to produce the natural insecticide Bacillus thuringiensis (Bt).

GM corn, sobeans and cotton planted in the US since 1996 have increased pesticide use…by about 50 million pounds.

HT crops allow broad-spectrum herbicides to be sprayed over growing plants, controlling weeds while leaving crops unharmed, making them popular with farmers. Despite increased seed prices, HT systems have become less expensive, largely because the price of herbicides containing glyphosate (Roundup) has fallen by half since HT crops were first introduced. (Crops tolerant to glyphosate, known as Roundup Ready varieties, are the largest share of acreage planted to HT crops).

Soybeans account for about 75% of the 400 million acres of HT crops and 54% of all GM acres that have been planted since 1996. While total pounds of pesticides applied to Bt corn and cotton have fallen modestly, the increase in herbicides applied on HT soybeans has been far greater. This, combined with the dominance of HT soybeans, has led to dramatic change in overall impact of GM crops on pesticide use.

Benbrook calculates the difference between the average pounds of pesticides applied on acres planted to GM crops, compared to the pounds applied to otherwise similar conventional crops. In their first three years of commercial sale (1996–1998), GM crops reduced pesticide use by about 25.4 million pounds, but in the last three years (2001–2003), over 73 million more pounds of pesticides were applied on GM acres.

The increase in overall pounds of pesticides applied across the three crops is due mainly to the need to apply more herbicides per acre planted to HT soybeans. USDA data show a marked increase in the per acre rate of glyphosate applied to HT soybeans between 2001 and 2002 from 0.85 pounds to 1.04 pounds.

This 22% jump was caused by a major price reduction in glyphosate, the need to control more difficult weeds, and the emergence of resistance and/or lessened sensitivity in weed species that were once fully controlled by one glyphosate application. So for HT soybeans, the difference in average herbicide pounds applied per acre between GM and conventional crops shifted from a reduction of 0.36 pounds per acre in 1996 to an increase of 0.47 pounds per acre in 2003.

The increase in pesticide use is no surprise, given that scientists had warned that heavy reliance on HT crops and a single herbicide…might lead to changes in weed communities and resistance.

Pesticide use estimates for 2003 in the report are preliminary, since USDA will not release these data until May 2004. However, estimates for 2003 are based on 2002 levels and trends in recent years. Benbrook has no doubt that average glyphosate application rates per acre of HT soybeans continued rising in 2003 due to the spread of glyphosate-tolerant marestail (horseweed), shifts in composition of weed communities toward species not as sensitive to glyphosate, early-stage resistance in some major weeds, and substantial price reductions and volume-based marketing incentives from competing manufacturers of glyphosate-based herbicides.

HT corn technology reduced herbicide use per acre from 1996 through 2001, but increased use thereafter. The difference in average herbicide pounds applied per acre between GM and conventional crops shifted from a reduction of 0.8 pounds per acre in 1996 to an increase of 0.58 pounds per acre in 2003.

The difference in herbicide application rates on HT and conventional cotton changed much like that of HT corn and soybeans, shifting from a reduction of 0.64 pounds in 1996 to an increase of 0.17 pounds per acre in 2003.

The report acknowledges that the other major category of GM crops—Bt corn and cotton—continues to reduce insecticide use by 2 million to 2.5 million pounds annually. The reduction in insecticide pounds applied per acre planted to Bt corn and cotton ranges from 0.33 pounds in 1996 to 0.06 pounds in 2003, and from 0.38 pounds in 1996 to 0.2 pounds in 2001–2003, respectively.

However, the increase in herbicide use on HT crops far exceeds the modest reductions in insecticide use on Bt crops, especially since 2001. The calculations also don’t take into account the volume of Bt toxin that is continuously expressed in the Bt crops’ plant cells. This amount is significant compared to the rates of application in today’s low-dose pesticides.

In short, over the last eight years, HT crops have increased pesticide use an estimated 70.2 million pounds, while Bt transgenic varieties have reduced pesticide use an estimated 19.6 million pounds. Thus, total pesticide use has risen some 50.6 million pounds over the eight-year period.

The increase in pesticide use is no surprise, given that scientists had warned that heavy reliance on HT crops and a single herbicide (in this case, glyphosate) for weed management might lead to changes in weed communities and resistance.

“For years weed scientists have warned that heavy reliance on herbicide tolerant crops would trigger ecological changes in farm fields that would incrementally erode the technology’s effectiveness. It now appears that this process began in 2001 in the United States in the case of herbicide tolerant crops,” said Benbrook.

…this rate of development suggests that new resistant biotypes will continue to arise.

According to Prof. Bob Hartzler, an extension weed management specialist from Iowa State University, glyphosate-resistant marestail in Roundup Ready soybeans first appeared in Delaware in 2000, spreading since as far west as Indiana, and identified in the Southeastern US where Roundup Ready cotton is grown. Other records of glyphosate-resistant weeds (not necessarily linked to HT crops) are rigid ryegrass in an orchard in Australia and in wheat production systems in Australia and California, Italian ryegrass in Chile and goosegrass in Malaysia.

Furthermore, waterhemp populations with individuals capable of surviving ‘normal’ user rates were identified in Iowa and Missouri the first year Roundup Ready soybeans were marketed. While Hartzler doesn’t think that waterhemp can as yet be considered glyphosate-resistant, the potential exists and should be closely monitored.

Since the first report of glyphosate-resistant rigid ryegrass in 1996, four additional resistant species with this trait have been identified. According to Harztler, this rate of development suggests that new resistant biotypes will continue to arise.

Prospects for GM crops leading to reduced pesticide use in the long term don’t bode well either. The pounds of herbicides required to achieve acceptable weed control is rising on most farms planting HT varieties, compared to the rates of application common between 1996–1998. In contrast, the amount of herbicides and insecticides applied per acre on conventional farms continues to trend downward as a result of incremental shifts toward newer low-dose pesticides and regulatory restrictions phasing out high-dose herbicides. As a result, the difference in total pounds of herbicides applied on HT versus conventional acres has increased steadily since 2000. This difference is likely to widen further if HT technology continues to be relied on as heavily as in recent years.